1. Field of the Invention
The present invention relates to a particulate size classification apparatus and method classifying sizes of nanoparticles to be used in the fields of luminescent materials, cosmetics, electronics, catalysts, and many others.
2. Description of the Related Art
Since particulates having sizes of the order of nanometers, so called nanoparticles, generally have a unique property presented due to size effects, or a large specific surface area, studies pursing their applications have been vigorously conducted in many areas in recent years. As examples of such applications, use of silicon nanoparticles as luminescent materials (see, for example, Non-Patent Document 1) or use of titanium oxide nanoparticles is known, and in any application, it is very important to control the sizes of nanoparticles.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2005-22886
[Non-Patent Document 1] T. Orii et al., Appl. Phys. Lett. 83 (2003)3395
[Non-Patent Document 2] Shouheng Sun et al., Science 287, 1989 (2000)
[Non-Patent Document 3] P. A. Baron, K. Willeke, Aerosol Measurements Principles, Techniques, and Applications, 2nd ed. Wiley, New York, 2001
[Non-Patent Document 4] Suzuki et al., APPLIED PHYSICS LETTER, VOL. 78, page 2043, 2001
Methods for producing nanoparticles may be classified mainly into methods with liquid phase systems and methods with gas phase systems, and the method using a reaction in a liquid phase system (see, for example, Non-Patent Document 2) has an advantage that particles have relatively uniform sizes, but raises a concern of existence of impurities regarding electric applications intended by the present inventor because the method uses surfactants, organic solvents and the like.
In the method using a reaction in a gas phase system, for example a laser ablation method or a plasma CVD (Chemical Vapor Deposition) method, clean particulates are generally obtained, but it is not easy to obtain particles having uniform sizes. For classifying the sizes of such particles, a differential mobility analyzer (DMA) is often used.
The DMA is an apparatus classifying particulates according to size using the electric mobility of particulates in a gas, and it is frequently used in areas of aerosols. The DMA is also often used in the subsidiary research institute of the inventor, since particle sizes can be made uniform satisfactorily when nanoparticles are classified using the DMA (see, for example, Patent Document 1).
However, the DMA is not perfect, and has a disadvantage that is considered fetal on some application areas. The disadvantage is that the amount of particulates (throughput) obtained through classification is very small particularly when the DMA is used for classification of nanoparticles.
Normally, in size classification by the DMA, it is necessary that particulates be charged, but it is difficult to charge nanoparticles, especially nanoparticles of 10 nm or less, with high efficiency, and therefore the aforementioned problem occurs. In this connection, the probability that particulates having sizes of, for example, 10 nm or less are obtained through size classification by the DMA is at most 2 to 3% of the amount of such particulates that actually exist. This raises a very serious problem when particulates having uniform sizes are put to practical use on a commercial basis.